To measure corner sharpness we photographed
the Enhanced Digital Camera Resolution Chart with each kit lens at its widest and longest focal lengths and with various
apertures selected in Aperture Priority mode.

The crops are taken from the lower
left corner of the chart and presented here at 100%. The 18mm crops show
a larger area of the chart because minimum focusing distances forced us
to shoot it from further away.

All three kit lenses are visibly soft in the extreme corners at wide angle with their apertures fully open. Closing the apertures improves matters across the board, most obviously on the Canon EF-S 18-55mm, although the Nikkor DX 18-70mm disappointingly still suffers beyond f8.
Zooming-in to telephoto shows the Nikkor still has some issues in the extreme corners, while the Canon and Sony lenses deliver reasonably good results, if a little low in contrast.

Note: this is a particularly tough test which concentrates entirely on the extreme corners of the frames. Examining areas even just a little further in can yield far superior results, especially for the Nikkor lens, which revealingly out-performs its rivals on outdoor tests when focused at infinity. That said, it's a good result here for the Canon EF-S 18-55mm.